1. Field of the Invention
The present invention relates to an optical fiber suitable for transmitting signals of a plurality of channels, a method of manufacturing the same, an optical communication system including the same and an optical fiber preform for obtaining the same in a wavelength division multiplexing (WDM) transmission.
2. Related Background Art
The WDM transmission system is an optical communication system which can realize a high-speed optical communication of a large capacity by transmitting signals of a plurality of channels. In the WDM transmission system, since the transmission loss of a silica-based optical fiber applied to an optical transmission line becomes small in the vicinity of the wavelength of 1.55 xcexcm and an optical amplifier which amplifies signals at the wavelength band of 1.55 xcexcm has been practically used, signals of a plurality of channels included in the wavelength band of 1.55 xcexcm are available.
In the optical transmission line through which signals of a plurality of channels propagate, it is known that when the chromatic dispersion is generated at the signal wavelength band (the wavelength band of 1.55 xcexcm), the pulse waveform of respective signals becomes broadened so that the transmission characteristics is deteriorated. Accordingly, from this point of view, it is desirable that the chromatic dispersion within the signal wavelength band is small. On the other hand, when the chromatic dispersion value within the signal wavelength band is approximately zero, a four-wave mixing which is one of the nonlinear optical phenomena is liable to be generated and hence, crosstalks and noises caused by the four-wave mixing are generated thus deteriorating the transmission characteristics. To suppress the generation of the four-wave mixing, the power of signals propagating through the optical transmission line may be reduced by making the repeater spacing short. However, it becomes necessary to install a large number of optical amplifiers along the whole optical transmission line thus pushing up a cost of the optical communication system as a whole.
To make the repeater spacing long while suppressing the occurrence of the above-mentioned nonlinear optical phenomenon, a dispersion-managed optical fiber in which portions having a positive chromatic dispersion and portions having a negative chromatic dispersion are alternately arranged, at a predetermined wavelength (for example, the wavelength being 1.55 xcexcm=1550 nm), has been proposed. In the optical transmission line which has adopted such a dispersion-managed optical fiber, the mean chromatic dispersion (at the wavelength of 1.55 xcexcm) from the viewpoint of the whole optical transmission line becomes approximately zero and hence, the deterioration of transmission characteristics caused by the generation of the chromatic dispersion can be effectively suppressed. Further, since the chromatic dispersion is generated in substantially all regions of the optical transmission line, the deterioration of transmission characteristics caused by the four-wave mixing can be effectively suppressed.
For example, Japanese parent Laid-open No. 201639/1996 discloses a dispersion-managed optical fiber which changes a sign (positive or negative) of the chromatic dispersion by changing the outer diameter of a core in the longitudinal direction. This publication also discloses a method of manufacturing such a dispersion-managed optical fiber. U.S. Pat. No. 5,894,537 discloses a dispersion-managed optical fiber which is designed such that signs (positive and negative) of the chromatic dispersions which are generated at respective portions are made different by changing the outer diameter of a core or the outer diameter of a cladding in the longitudinal direction, and it also discloses a method of manufacturing such a dispersion-managed optical fiber. The Japanese Patent Laid-open No. 318824/1997 discloses an optical fiber cable in which two kinds of optical fibers which differ from each other in their effective areas as well as in signs (positive and negative) of the chromatic dispersion are connected.
Upon reviewing the conventional dispersion-managed optical fiber and cable, the inventors of the present invention have found following problems. That is, the conventional dispersion-managed optical fiber disclosed in the Japanese Patent Laid-open No. 201639/1996 and U.S. Pat. No. 5,894,537 is manufactured by drawing the optical fiber preform which changes the outer diameter of the core or the outer diameter of the cladding along the longitudinal direction and hence, the manufacturing is not easy. Further, in the conventional dispersion-managed optical fiber, since the outer diameter of the core or the outer diameter of the cladding is changed along the longitudinal direction, it is difficult to connect this optical fiber with other optical fiber. Further, there arises a case that the connection loss becomes large. For example, the optical fiber cable disclosed in Japanese Patent Laid-open No. 318824/1997 connects two kinds of optical fibers which differ from each other in the effective area and hence, the connection loss becomes large.
The present invention has been made to solve the above-mentioned problems and it is an object of the present invention to provide a dispersion-managed optical fiber having a structure which facilitates the manufacturing thereof and the connection thereof with other optical fiber, a method of manufacturing such a dispersion-managed optical fiber, an optical communication system which adopts the dispersion-managed optical fiber as an optical transmission line, and an optical fiber preform for obtaining the dispersion-managed optical fiber.
A dispersion-managed optical fiber according to the present invention is a silica-based optical fiber ensuring its single mode at a predetermined wavelength within a signal wavelength band, that is, a continuous (including unitary) optical fiber in which one or more first portions having a positive chromatic dispersion at the predetermined wavelength and one or more second portions having a negative chromatic dispersion at the predetermined wavelength are arranged alternately and adjacent to each other.
This dispersion-managed optical fiber includes a plurality of glass layers which are sequentially laminated in a radial direction. Among the plurality of glass layers, a dopant concentration of a glass layer doped with a dopant for adjustment of refractive index is made uniform such that the maximum change along the longitudinal direction of the dispersion-managed optical fiber is not more than 20-30%, and preferably, not more than 10%. Further, a refractive index of a glass layer which does not substantially include GeO2 as a dopant is changed along the longitudinal direction of the dispersion-managed optical fiber. Further, the dispersion-managed optical fiber according to the present invention may have a constitution in which the stresses remaining in the plurality of glass layers may be changed along the longitudinal direction of the dispersion-managed optical fiber. It is preferable that a core region in the dispersion-managed optical fiber includes a layer comprised of non-intentionally-doped glass (hereinafter called xe2x80x9cpure silica glassxe2x80x9d). This is because that since the viscosity of the pure silica glass layer is greater than that of the glass layer including a dopant, the adjustment of residual stress is facilitated. Further, even when a predetermined amount of GeO2 is added into this pure silica glass layer unintentionally during the manufacturing, the relative refractive index difference of this layer, in which the residual stress is given, with respect to the pure silica glass is restricted to a value lower than the relative refractive index difference of the glass layer into which an amount of GeO2 equal to that of the added GeO2 is added, and therefore the influence of addition of GeO2 can be effectively suppressed. In this specification, the relative refractive index difference of each glass layer to a reference region is given by the equation (nxe2x88x92n0)/n0 (order being irregular) where n indicates the refractive index of each glass layer and n0 indicates the refractive index of the reference region and they are expressed by percentage. Accordingly, when the pure silica glass is used as the reference, the relative refractive index difference of the glass layer having the lower refractive index than the pure silica glass takes the negative value and the relative refractive index difference of the glass layer having the higher refractive index than the pure silica glass takes the positive value.
As has been described heretofore, in the dispersion-managed optical fiber according to the present invention, the dopant concentration is held in the uniform state along the longitudinal direction of the dispersion-managed optical fiber and the refractive index or the residual stress of the glass layer which is not doped with GeO2 is changed along the longitudinal direction of the dispersion-managed optical fiber. Due to such a structure, without changing the cross-sectional dimension of the dispersion-managed optical fiber along the longitudinal direction, the continuous dispersion-managed optical fiber in which the portions having a positive chromatic dispersion at the predetermined wavelength and the portions having a negative chromatic dispersion at a predetermined wavelength are arranged alternately can be obtained. Accordingly, the manufacturing of the dispersion-managed optical fiber according to the present invention is facilitated and there is no possibility that the connection of such optical fiber with other optical fiber increases the connection loss.
It is preferable to set the signal wavelength band to 1.53 xcexcm-1.60 xcexcm and it is more preferable to set the signal wavelength band to 1.54 xcexcm-1.56 xcexcm. This is because that, in general, such a wavelength band is a range which allows the silica-based optical fiber to suppress the transmission loss as small as possible and a sufficient transmission quality can be maintained in the WDM transmission which adopts the dispersion-managed optical fiber as an optical transmission line.
In the dispersion-managed optical fiber according to the present invention, it is preferable that each first portion has the chromatic dispersion of not less than +1 ps/nm/km but not more than +10 ps/nm/km at the predetermined wavelength within the signal wavelength band and each second portion has the chromatic dispersion of not less than xe2x88x9210 ps/nm/km but not more than xe2x88x921 ps/nm/km at the predetermined wavelength within the signal wavelength band. Further, it is preferable that each first portion has the length of not less than 500 m but not more than 10 km and each second portion has the length of not less than 500 m but not more than 10 km. By designing the first and second portions such that they fall within the above-mentioned ranges, the easiness of manufacturing the dispersion-managed optical fiber can be ensured and simultaneously the deterioration of transmission characteristics caused by the interaction between the cumulative chromatic dispersion and the nonlinear optical phenomenon can be effectively suppressed.
The first portion has the positive dispersion slope at the predetermined wavelength within the signal wavelength band and the second portion has the negative dispersion slope at the predetermined wavelength within the signal wavelength band. Due to such a constitution, the increase of cumulative chromatic dispersion can be effectively suppressed and simultaneously the cumulative dispersion slope from a viewpoint of the whole dispersion-managed optical fiber can be made small. Further, the wider band can be used as the signal wavelength band for the WDM transmission.
In the dispersion-managed optical fiber according to the present invention, it is preferable that the cumulative length of transient portions, which are positioned between each first portion and each second portion arranged alternately and adjacent to each other and which have the chromatic dispersion whose absolute value is less than 1 ps/nm/km at the predetermined wavelength in the signal wavelength band amounts to not more than 10% of the total length of the dispersion-managed optical fiber. In this case, by designing such that the transient portions which are liable to generate the nonlinear optical phenomenon become short, the deterioration of the transmission characteristics caused by the nonlinear optical phenomenon can be effectively suppressed.
Further, in the dispersion-managed optical fiber according to the present invention, the mean chromatic dispersion at the predetermined wavelength within the signal wavelength band from the viewpoint of the whole dispersion-managed optical fiber has the absolute value of not more than 3 ps/nm/km and preferably of substantially 0 (xe2x88x921 to +1 ps/nm/km). Due to such a constitution, at the predetermined wavelength in the signal wavelength band, the cumulative chromatic dispersion of the whole dispersion-managed optical fiber can be suppressed to a small amount so that the deterioration of transmission characteristics caused by the interaction between the cumulative chromatic dispersion and the nonlinear optical phenomenon can be effectively suppressed. It is preferable that the effective area at the predetermined wavelength in the signal wavelength band is not less than 40 xcexcm2. It is also preferable that the polarization mode dispersion is not more than 0.2 psxc2x7kmxe2x88x92xc2xd. In both cases, the deterioration of the transmission characteristics caused by the nonlinear optical phenomenon and the polarization mode dispersion can be effectively suppressed.
The dispersion-managed optical fiber according to the present invention includes a core region which extends along a predetermined axis and a cladding region provided around an outer periphery of the core region. Particularly, the core region preferably includes a layer substantially provided of pure silica glass. In the layer provided of pure silica glass, the residual stress which is generated by the drawing largely depends on the drawing tension so that the refractive index is changed corresponding to this residual stress and also the chromatic dispersion is also changed corresponding to this residual stress. Accordingly, such a layer is preferable to realize the dispersion-managed optical fiber. Further, following refractive index profiles are applicable to the dispersion-managed optical fiber according to the present invention.
That is, the first refractive index profile is realized by the core region comprising a first core, a second core and a third core, and a cladding region provided around the outer periphery of the core region. Particularly, the first core is a glass layer doped with GeO2 and has the relative refractive index difference of not less than 0.4% with respect to the reference region within the cladding region. The second core is a glass layer provided around the outer periphery of the first core and doped with F element. The second core has the refractive index lower than that of pure silica glass. The third core is a glass layer provided around the outer periphery of the second core and substantially is comprised of pure silica glass. The cladding region includes a layer doped with F element and having the refractive index lower than that of pure silica glass. The first core preferably has the outer diameter of not less than 4 xcexcm but not more than 9 xcexcm and the relative refractive index difference of not less than 0.4% but not more than 1.1% to the reference region in the cladding region. The second core preferably has the outer diameter of not less than 6 xcexcm but not more than 20 xcexcm and the relative refractive index difference of not less than 0% but not more than 0.1% to the reference region in the cladding region. The third core preferably has the outer diameter of not less than 10 xcexcm but not more than 30 xcexcm and the relative refractive index difference of not less than 0.05% but not more than 0.5% to the reference region in the cladding region.
The second refractive index profile differs from the first refractive index profile on a point that the refractive index of the second core is lower than the refractive index of the F element doped layer in the cladding region. Here, the first core has the outer diameter of not less than 4 xcexcm but not more than 9 xcexcm and the relative refractive index difference of not less than 0.4% but not more than 1.1% to the reference region in the cladding region. The second core has the outer diameter of not less than 6 xcexcm but not more than 20 xcexcm and the relative refractive index difference of not less than xe2x88x920.6% but less than 0% to the reference region in the cladding region. The third core has the outer diameter of not less than 10 xcexcm but not more than 30 xcexcm and the relative refractive index difference of not less than 0.05% but not more than 0.5% to the reference region in the cladding region.
Further, the third refractive index profile is realized by a core region comprising a first core and a second core extending along a predetermined axis and a cladding region provided around the outer periphery of the core region. Here, the first core is a glass layer doped with GeO2 and has the relative refractive index difference of not less than 0.7% with respect to the reference region in the cladding region. The second core is a glass layer provided around the outer periphery of the first core and is substantially comprised of pure silica glass. The cladding region includes a layer doped with F element and having the refractive index lower than that of pure silica glass. The first core preferably has the outer diameter of not less than 3 xcexcm but not more than 6 xcexcm and the relative refractive index difference of not less than 0.7% but not more than 1.2% to the reference region in the cladding region. The second core preferably has the outer diameter of not less than 15 xcexcm but not more than 25 xcexcm and the relative refractive index difference of exceeding 0% but not more than 0.3% to the reference region in the cladding region.
In any one of the first to third refractive index profiles, the cladding region may comprises an inner cladding provided around the outer periphery of the core region and an outer cladding provided around the outer periphery of the inner cladding and having the refractive index higher than that of the inner cladding (depressed cladding structure). In such a depressed cladding structure, the inner cladding preferably has the outer diameter of not less than 25 xcexcm but not more than 60 xcexcm and the reference refractive index difference of not less than xe2x88x920.4% but less than 0% with respect to the outer cladding (the reference region of the cladding region).
In this manner, there may be a case that the cladding region comprises a plurality of glass layers which differ in the refractive index. Accordingly, when the cladding region comprises a single glass layer, the cladding region itself becomes the reference region and when the cladding region has the depressed cladding structure, the outer cladding as the outermost layer becomes the reference region.
Further, the fourth refractive index profile is realized by a single core region and a cladding region provided around the outer periphery of the core region. The core region is a glass layer substantially comprised of pure silica glass. The cladding region includes an inner cladding provided around the outer periphery of the core region and doped with F element and an outer cladding provided around the outer periphery of the inner cladding and being a glass layer doped with F element and having the refractive index higher than that of inner cladding. The core region preferably has the outer diameter of not less than 3 xcexcm but not more than 7 xcexcm and the relative refractive index difference of not less than 0.4% but not more than 0.9% to the outer cladding (reference region in the cladding region). The inner cladding preferably has the outer diameter of not less than 7 xcexcm but not more than 14 xcexcm and the relative refractive index difference of not less than xe2x88x920.6% but less than 0% with respect to the outer cladding.
The outer diameter of the dispersion-managed optical fiber according to the present invention may be changed in synchronous with the change of the refractive index of a glass layer not doped with GeO2 along the longitudinal direction of the dispersion-managed optical fiber. Further, the outer diameter of the dispersion-managed optical fiber according to the present invention may be changed in synchronous with the change of the residual stress in each glass layer along the longitudinal direction of the dispersion-managed optical fiber. In both cases, due to the change of the outer diameter of the fiber, the adjustment of chromatic dispersion can be facilitated. Further, even when the outer diameter of the fiber is changed, the adjustment of chromatic dispersion can be performed effectively with a slight change of the outer diameter of the fiber. Since the sufficient dispersion adjustment effect can be obtained with the slight change of the outer diameter of the fiber, the dispersion-managed optical fiber can be manufactured easily and the increase of the connection loss in connecting the dispersion-managed optical fiber to other optical fiber can be effectively suppressed.
Subsequently, the dispersion-managed optical fiber according to the present invention is obtained in the following manner. That is, a method of manufacturing the dispersion-managed optical fiber according to the present invention prepares a predetermined optical fiber preform and draws this optical fiber preform while adjusting the drawing tension. The prepared optical fiber preform is made uniform such that the maximum change of dopant concentration along the longitudinal direction of the optical fiber preform in the region which contains the dopant for adjustment of refractive index among regions corresponding to the plurality of glass layers in the dispersion-managed optical fiber becomes not more than 20%-30%, and preferably, not more than 10%. Further, the prepared optical fiber preform may be made uniform such that the maximum change of refractive indices of respective regions corresponding to the plurality of glass layers in the dispersion-managed optical fiber to the pure silica glass along the longitudinal direction of the optical fiber preform becomes not more than 20%-30%.
To ease the manufacturing of the dispersion-managed optical fiber, it is preferable that the drawing tension applied to the prepared optical fiber preform is changed by changing the temperature of molten portion of the optical fiber preform or changing the drawing speed. In addition, the outer diameter of the fiber may be changed in synchronous with the change of the drawing speed (the temperature change of the molten portion in the optical fiber preform and the change of drawing speed). By changing the outer diameter of the fiber, the adjustment of chromatic dispersion is facilitated. Further, even when the outer diameter of the fiber is changed, the adjustment of chromatic dispersion can be sufficiently performed with the slight change of the outer diameter of the fiber.
The dispersion-managed optical fiber according to the present invention is applicable to the optical communication system for the WDM transmission. The dispersion-managed optical fiber constitutes a part of an optical transmission line installed between repeaters, such as between a transmitter which irradiates signals of a plurality of channels and a repeater which includes an optical amplifier, between respective repeaters or between a repeater and a receiver. Particularly, it is preferable that the dispersion-managed optical fiber is arranged at the upstream side as seen from the traveling direction of the signals having a wavelength within the signal wavelength band in the repeater spacing. By arranging the dispersion-managed optical fiber in this manner, the deterioration of the transmission characteristics can be effectively suppressed at the upstream of the repeater spacing where the signal power is large and hence, the nonlinear optical phenomenon is liable to be generated.
Further, in each repeater spacing of the optical communication system according to the present invention, at the predetermined wavelength within the signal wavelength band, it is preferable that the absolute value of the mean chromatic dispersion from the viewpoint of the whole repeater spacing is not more than 3 ps/nm/km and further substantially 0 (xe2x88x921 to +1 ps/nm/km). This is because that the deterioration of transmission characteristics caused by the interaction between the cumulative chromatic dispersion in the optical transmission line and the nonlinear optical phenomenon can be effectively suppressed and the sufficient transmission quality of the WDM transmission can be maintained. Further, to realize the soliton communication, in each repeater spacing of the optical communication system, it is preferable that the mean chromatic dispersion at the predetermined wavelength within the signal wavelength band is not less than 0.1 ps/nm/km but not more than 1.0 ps/nm/km.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.